As wireless communication systems such as cellular telephone, satellite, and microwave communication systems become widely deployed and continue to attract a growing number of users, there is a pressing need to accommodate a large and variable number of communication subsystems transmitting a growing volume of data with a fixed resource such as a fixed channel bandwidth accommodating a fixed data packet size. Traditional communication system designs employing a fixed resource (e.g., a fixed data rate for each user) have become challenged to provide high, but flexible, data transmission rates in view of the rapidly growing customer base.
The third generation partnership project long term evolution (“3GPP LTE”) is the name generally used to describe an ongoing effort across the industry to improve the universal mobile telecommunications system (“UMTS”) for mobile communications. The improvements are being made to cope with continuing new requirements and the growing base of users, and higher data rates and higher system capacity requirements. Goals of this broadly based project include improving communication efficiency, lowering costs, improving services, making use of new spectrum opportunities, and achieving better integration with other open standards and backwards compatibility with some existing infrastructure that is compliant with earlier standards.
The wireless communication systems as described herein are applicable to, for instance, 3GPP LTE compatible wireless communication systems and of interest is an aspect of LTE referred to as “evolved UMTS Terrestrial Radio Access Network,” or E-UTRAN and also UTRAN communications systems. In E-UTRAN systems, the e-Node B may be, or is, connected directly to the access gateway (“aGW,” sometimes referred to as the services gateway “sGW”). Each Node B may be in radio contact with multiple UEs (generally, user equipment including mobile transceivers or cellphones, although other devices such as fixed cellular phones, mobile web browsers, laptops, PDAs, MP3 players, and gaming devices with transceivers may also be UEs) via the radio Uu interface.
In the present discussion, particular attention is paid to enhancements presently being considered for Release 9 and Release 10 (sometimes referred to as “LTE Advanced”) of the 3GPP standards. These future evolutions of LTE will have additional requirements and demands for increased throughput. Although the discussion uses E-UTRAN as the primary example, the application is not limited to E-UTRAN, LTE or 3GPP systems. In general, E-UTRAN resources are assigned more or less temporarily by the network to one or more UEs by use of allocation tables, or more generally by use of a downlink resource assignment channel or physical downlink control channel (“PDCCH”). The PDCCH is used to allocate resources in other channels, including the physical downlink shared channel (“PDSCH”). LTE is a packet-based system and, therefore, there may not be a dedicated connection reserved for communication between a UE and the network. Users are generally scheduled on a shared channel every transmission time interval (“TTI”) by a Node B or an evolved Node B (“e-Node B”). A Node B or an e-Node B controls the communications between user equipment terminals in a cell served by the Node B or e-Node B. In general, one Node B or e-Node B serves each cell. An e-Node B may serve multiple cells. A Node B or e-Node B may be referred to as a “base station.” Resources needed for data transfer are assigned either as one time assignments or in a persistent/semi-static way.
The LTE-A, also referred to as 4G, generally supports a large number of users per cell with quasi-instantaneous access to radio resources in the active state. The goals for LTE-A which is currently being developed as an implementation standard are to increase bandwidth to 1 Gbps for low mobility situations, and 100 Mbps for higher mobility situations. More generally, the goal for LTE-A and 4G is to provide a mobile device user with the same performance as, or similar performance to, a fixed station fiber optic internet connection. As proposed, LTE-A systems are to be backwards compatible with LTE Release 8 equipment so that the existing user equipment and eNBs (Release 8 and Release 9) can operate in LTE-A network. Similarly, LTE-A user equipment should be able to operate in existing (Release 8 and Release 9) networks.
In LTE and LTE-A systems, in order to facilitate scheduling on the shared channel, the e-Node B transmits a resource allocation to a particular UE in a physical downlink control channel (PDCCH) to the UE. The grant allocation information may be related to both uplink and downlink resources. The allocation information may include information about which physical resource blocks in the frequency domain are allocated to the scheduled user(s), the modulation and coding schemes to use, what the size of the transport block is, and the like.
The lowest layer of communication in the UTRAN or e-UTRAN system, Layer 1, is implemented by the Physical Layer (“PHY”) in the UE and in the Node B or e-Node B. The PHY performs the physical transport of the packets between the UE and eNB over the air interface using radio frequency signals. In order to ensure a transmitted packet was received, an automatic retransmit request (“ARQ”) and a hybrid automatic retransmit request (“HARQ”) approach is provided. Thus, whenever the UE receives packets through one of several downlink channels, including dedicated channels and shared channels, the UE performs a communications error check on the received packets, typically a Cyclic Redundancy Check (“CRC”), and in a later subframe following the reception of the packets, transmits a response on the uplink channel to the e-Node B or base station. The UE response is either an Acknowledgment (“ACK”) or a Non Acknowledgment (“NAK”) message. If the response is a NAK, the e-Node B automatically retransmits the packets in a later subframe on the downlink (“DL”). In the same manner, any uplink (“UL”) transmission from the UE to the e-Node B is responded to, at a specific subframe later in time, by a ACK/NAK message on the DL channel to complete the HARQ. In this manner, the packet communications system remains robust with a low latency time and fast turnaround time.
In order to accomplish the system performance goals of LTE-A, aggregation of component carriers (CC) is proposed. By aggregating the CCs, the necessary bandwidth to obtain the performance is obtained.
An aspect of the ACK/NAK signaling in the uplink channels by the UE to the eNB is that the UE cannot acknowledge missed DL grants that the UE is not aware of. That is, the UE only knows whether it receives the DL grants that it recognizes. A missed DL grant that is the last DL grant in a resource allocation on the PDCCH is not detectable by the UE. In order to make sure the eNB and UE share their understanding of the grants allocated in LTE systems, a downlink assignment index (DAI) is transmitted on the downlink. Reference is made to the 3GPP Technical Specification numbered 3GPP TS 36.213, entitled “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 8)”; Version 8.7.0, dated 05-2009, available from the 3gpp at the website URL www.3gpp.org; which is hereby incorporated by reference herein in its entirety. Chapter 7.3 of the TS 36.213 describes the DAI encoding for LTE Release 8. A two bit DAI field is included in the downlink grant transmitted to the UE on PDCCH. This information is made available to avoid higher layer errors in case of a downlink grant failure.
The 2-bit DAI is encoded. In LTE Release 8, a “pure counter” encoding scheme is used. This encoded DAI is transmitted to the UE so that the UE knows when a DL grant is missed. Without this information, the UEs ACK/NAK information provided on the uplink channel (PUCCH, for example) would be incomplete. In addition to the ACK/NAK signals, the UE may transmit other signals on the PUCCH. For LTE Release 8 devices, these control signals may include the channel quality indicator (CQI), scheduling request indicator (SR), the precoding matrix indicator (PMI) and so forth. Unless the UE has the DAI and knows how many DL grant allocations were expected, the ACK/NAK information transmitted by the UE as part of the uplink message may be misinterpreted by the receiving eNB. The use of the DAI field mitigates this possibility. The UE can know when to provide NAK or sometimes discontinuous transmission (DTX) information in the appropriate places in the UL message, so that the eNB and the UE both understand what happened in the previous downlink message cycle.
As presently proposed, LTE-A systems will apply the PUCCH of LTE Release 8 communications to transmit control signals. The PUCCH signals include ACK/NAK, CQI, and SR, for example. In LTE Release 8 for TDD, the UE can report ACK/NAK results for multiple DL subframes during one UL subframe. The ACK/NAK signaling for multiple DL subframes can be made using two modes; either ACK/NAK bundling or ACK/NAK multiplexing modes. The reader is again referred to the TS 36.213 Chapter 7.3 for more information.
In ACK/NAK bundling mode, the ACK/NAK bits are first bundled in the time domain to get one bit (for example, ACK/NAK bundling is a logical AND operation on the previous ACK/NAK bits) and the bundled bits are modulated and transmitted on the PUCCH, which is associated with the last detected DL grant.
In ACK/NAK multiplexing, a channel selection for PUCCH is used which enables transmission of 2-4 bits via a single PUCCH channel. The selected channel and the quadrature phase shift keyed (QPSK) constellation point used are determined based on the ACK/NAK/DTX states corresponding to multiple DL subframes as shown in Tables 10.1-2, 10.1-3 and 10.1-4 of 3GPP TS 36.213.
In order to reach the system performance goals of the LTE-A proposals, LTE-A adds the use of CC aggregation. This added feature introduces another degree of freedom for ACK/NAK feedback signaling that is transmitted on the PUCCH. A consequence of this extra degree of freedom is that more ACK/NAK bits, for example up to 20 bits for a case using 5 CCs in aggregate, are needed to be transmitted during one uplink subframe.
Problems associated with the LTE-A methods for ACK/NAK reporting on PUCCH in TDD include how to provide proper ACK/NAK feedback supporting CC aggregation; how to perform the performance trade-off between downlink throughput and UL coverage, that is, how to configure different feedback signaling profiles for multiple ACK/NAK bits to be signaled in the uplink; how to avoid certain error cases related to the configurable ACK/NAK feedback, and how to encode the DAI field for LTE-A with CC aggregation.
Notably, the use of the LTE Release 8 DAI encoding method for each CC, which is a straightforward approach, will result in additional error cases. These will occur with the failure of the last N consecutive DL allocation grants on each CC. So this simple approach will not be sufficient. A more comprehensive approach is needed.
To date, a method and system for efficiently implementing the ACK/NAK support needed for the physical uplink control channel (PUCCH) transmissions relating to the prior physical downlink control channel (PDCCH) transmission in a UE using the aggregated CCs of LTE-A has not been determined. Various error cases that would occur if prior art approaches are used necessitate a new method and system for ACK/NAK uplink messages in the LTE-A PUCCH channel.
A need thus exists for methods and systems to efficiently support the use of ACK/NAK uplink messages on the PUCCH channels for TDD UEs, when LTE-A aggregate component carrier capability is utilized, without the problems of the prior art approaches.